Extrusion molding of ceramics has been performed by passing a ceramic green body or composition, which is obtained by mixing and kneading molding aids such as organic binders, surfactants, lubricants, and plasticizers with ceramic materials, through dies having a desired shape into a sheet, a bar, a hollow tube, a rectangular column, a hollow rectangular column, or a honeycomb structure. In particular, the extrusion-molded body in the form of ceramic honeycombs has been in use as a carrier for exhaust gas cleaning catalyst, a filter, and a heat exchanger in the fields of automobiles and various industries. According to the revision of the recent exhaust gas regulations, the partition walls of the honeycomb structure has been thinner so as to improve the cleaning performance, reduce a pressure loss and improve heat exchange efficiency.
In related art, compositions for ceramic extrusion-molded bodies have entirely made use of cellulose ethers as an organic binder because of their excellent plasticity, water retention and thermal gelation characteristics.
However, these cellulose esters are disadvantageous in that they increase in frictional force with the die portion during extrusion molding and thus, the extrusion temperature rises owing to this frictional resistance. Eventually, the cellulose ether in the extrusion molding composition is thermoreversibly gelled, so that fluidity of a plasticizer becomes deteriorated to increase a molding pressure. The resulting ceramic extrusion-molded body cannot be quickly forced out from the extrusion molding dies. In general, with the extrusion molding of a composition to which an organic binder exhibiting no thermal gelation is added, the viscosity relying on the organic binder lowers when the discharge temperature is increased, and thus the molding speed can be made faster. In molding the ceramic extrusion-molded body using a thermoreversibly gelling cellulose ester as an organic binder, there has been a problem in that it is difficult to increase the molding speed.
In order to improve extrusion moldability for overcoming these disadvantages, investigations have been made using a variety of organic additives as proposed in JP-A 11-58335 and JP-A 2002-293645 and satisfactory results have not been obtained yet.